1. Field of the Invention
The present invention relates to an image input apparatus such as a digital camera or image scanner, and a method of processing input image data input to the image input apparatus.
2. Related Background Art
An image input apparatus as shown in FIG. 1 is conventionally known. This image input apparatus includes a lens 101, a solid-state image pickup element 102, and a shutter (movable shielding member) 103 for controlling incident light to the solid-state image pickup element 102. Under the control of a CPU 108, a shutter driving device 104 controls opening/closure of the shutter 103, and an image pickup element driving circuit 105 controls driving of the solid-state image pickup element 102.
In this image input apparatus, when the shutter driving device 104 opens the shutter 103, an image of an object is formed on the solid-state image pickup element 104 via the lens 101.
The image pickup element driving circuit 105 has an operation mode control function for setting an accumulation mode in which optical information on the solid-state image pickup element 102 is converted into an analog electrical signal and accumulated, and a read-out mode in which the accumulated image pickup data (analog electrical signal) is read out. The analog electrical signal accumulated in the solid-state image pickup element 102 and read out from it is converted into a digital electrical signal by an A/D converter 106, stored in a memory 107, and processed by an image processor (not shown).
FIG. 2 is a timing chart showing the relationship between the open/close timing of the shutter and the operation mode of the solid-state image pickup element.
First, the operation mode of the solid-state image pickup element 102 is set to the accumulation mode with the shutter 103 closed, and the shutter 103 is opened for a predetermined time s. When this predetermined time s has elapsed, the shutter 103 is closed. After that, the solid-state image pickup element 102 is set to the read-out mode by the image pickup element driving circuit 105 to perform an image pickup data read-out process. The readout image data is stored in the memory and subjected to image processing.
When a CMOS sensor is used as the solid-state image pickup element 102, as shown in FIG. 3, the image pickup element driving circuit 105 outputs an all-pixel reset pulse to remove electric charge accumulated in all pixels of the solid-state image pickup element 102. At the same time, the operation mode is set to the accumulation mode to start accumulating new optical information.
It is known that the solid-state image pickup element 102 has noise so-called fixed pattern noise. It is also known that in order to remove this fixed pattern noise, black image data is subtracted from image data, and image processing is performed on the basis of this subtracted data.
FIG. 4 is a timing chart showing the relationship among the open/close timing of the shutter, the generation timing of the reset pulse, and the operation mode of the solid-state image pickup element when the above subtraction is performed.
Similar to FIG. 3, the solid-state image pickup element 102 is set to the accumulation mode at the same time the all-pixel reset pulse is generated. The shutter 103 is opened, and then closed when the predetermined time s has elapsed. Subsequently, the operation mode is switched to the read-out mode to read out image data. That is, image data of an object is accumulated by setting an accumulation mode time t to be slightly longer than the predetermined time s during which the shutter 103 is opened. When the shutter 103 is closed, the operation mode of the solid-state image pickup element 102 is changed to the read-out mode to read out the image data. The readout image data is stored in the memory 107.
When the image data read-out process is completed, another all-pixel reset pulse is generated, and at the same time the solid-state image pickup element 102 is set to the accumulation mode. That is, the operation mode of the solid-state image pickup element 102 is set to the accumulation mode for the same time period (accumulation mode time t) as the accumulation mode when the shutter is open, thereby accumulating black image data. When the accumulation mode time t has elapsed, the operation mode is switched to the read-out mode to read out the black image data. The readout image data is stored as black image data in the memory 107. After that, this black image data is subtracted from the image data obtained when the shutter is open, and image processing is performed on the basis of the difference. Consequently, a high-quality image from which the fixed pattern noise is removed can be obtained.
When the exposure time is short, as described above, this conventional image input apparatus can remove the fixed pattern noise and thereby obtain high-quality image data. However, if the exposure time is relatively long, the apparatus cannot control image data.
More specifically, a so-called dark current generally exists in an image pickup element, and the fixed pattern noise described above is presumably principally caused by the nonuniformity of this dark current. In addition, this dark current increases in proportion to the time of accumulation to the image pickup element.
In the conventional image input apparatus, therefore, in case of that the exposure time is short the fixed pattern noise can be removed by subtracting black image data from image data of an object. However, in case of that the exposure time is long, the amount of dark current increases, and this increases the level of black image data. This may narrow the dynamic range of image data.
Also, when photographing is performed for a long time by using a solid-state image pickup element 102 having a large dark current or when photographing is performed for a long time exceeding the capability of the solid-state image pickup element 102, black image data alone exceeds the input range of the A/D converter  106. This makes the user totally unable to obtain desired image data.
Furthermore, the above conventional image pickup element acquires the black image data only once. Hence, a random noise component when this black image data is acquired is added as fixed pattern noise of image data, leading to deterioration of the S/N ratio.